Oxidation of low-density lipoprotein (LDL) is recognized to be a key step in atherogenesis. Previous studies show that LDL contains low molecular weight antioxidants such as vitamin E, beta-carotene, and ubiquinol, that can retard oxidative modification. In this report, we have evaluated the antioxidant potential of apolipoprotein B-100 (apo-B) thiols during LDL oxidation. Both apo-B thiols and vitamin E were depleted concomitantly during the lag phase of Cu(2+)-mediated LDL oxidation. The rate of thiol depletion was significantly inhibited by the lipophilic spin trap N-tert-butyl-alpha-phenylnitrone (PBN) but not by the water-soluble spin trap alpha-(4-pyridyl-1-oxide)-N-tert-butylnitrone (POBN). Blocking apo-B thiols with sulfhydryl modifying agents increased the oxidizability of LDL. As with Cu(2+), peroxynitrite also caused depletion of apo-B thiols, and again thiol depletion was inhibited by PBN but not by POBN. A PBN-lipid-derived radical adduct was observed by the electron spin resonance (ESR) technique during oxidation of LDL with peroxynitrite. We conclude that apo-B thiol depletion is mediated by lipid peroxidation, prior to the onset of the propagation phase of LDL oxidation. The implications of apo-B thiols as intrinsic antioxidants of LDL are discussed.